1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same. More particularly, the invention relates to a photographic optical system such as a single-lens reflex camera, a digital still camera, a digital video camera, a TV camera, and a monitoring camera, for example.
2. Description of the Related Art
An image pickup apparatus such as a single-lens reflex camera or a video camera is required to be capable of fast and high-precision automatic focusing. A rear-focus zoom lens is known as being capable of easily performing fast automatic focusing. The rear-focus zoom lens performs focusing by moving a small, lightweight lens unit other than a first lens unit at the object side.
A recent single-lens reflex camera has a moving image capturing function and is required to be capable of automatic focusing during moving image capturing. The high-frequency detection system (TV-AF system) is often used as an automatic focusing system to capture a moving image. The high-frequency detection system detects high-frequency components in an image capture signal to evaluate an in-focus state of the photographic optical system.
An image pickup apparatus using the TV-AF system wobbles or fast vibrates a focus lens unit in the optical axis direction to detect a direction of deviation from the in-focus state. After the wobbling, the image pickup apparatus detects signal components corresponding to a specific frequency band for an image region in an output signal from an image sensor to calculate an optimal position of the focus lens unit for the in-focus state. The image pickup apparatus then moves the focus lens unit to complete the focusing. The focus lens unit needs to be small and lightweight so that the focus lens unit can be wobbled. A known zoom lens includes lens units part of which is a small and lightweight lens unit for focusing.
As discussed in U.S. Pat. No. 7,184,221, a six-unit zoom lens includes first through sixth lens units having respective negative, positive, negative, positive, negative, and positive refractive powers. The small, lightweight fifth lens unit performs focusing.
As discussed in U.S. Patent Application Publication No. 2010/0091170, a zoom lens includes first through fifth lens units having respective positive, negative, negative, positive, and positive refractive powers. The third lens unit performs focusing.
As discussed in U.S. Patent Application Publication No. 2010/0091171, a zoom lens includes first through fifth lens units having respective positive, negative, positive, positive, and positive refractive powers. The third lens unit performs focusing.
As discussed in U.S. Pat. No. 6,556,356, a zoom lens includes first through fourth lens units having respective positive, negative, positive, and positive refractive powers. The second lens unit performs focusing.
As discussed in Japanese Patent Application Laid-Open No. 2001-033697, a zoom lens includes first through fifth lens units having respective positive, negative, positive, negative, and positive refractive powers. The fourth lens unit performs focusing.
The focus lens unit can use a small, lightweight lens unit having a small number of lenses to improve the automatic focus speed. Further, the focus lens unit can use a lens unit having high refractive power to reduce the amount of movement during focusing. If the focus lens unit includes a small number of lenses, however, increasing the refractive power for the focus lens unit generally increases residual aberration for the focus lens unit and increases an aberration variation due to focusing. Accordingly, the power for the focus lens unit cannot be fully increased.
By contrast, decreasing the power for the focus lens unit increases the amount of movement of the focus lens unit during focusing. As a result, the space for the focus lens unit to move increases to unsuccessfully ensure a sufficient space for a zooming lens unit to move. Miniaturizing the entire zoom lens is difficult while high optical performance is maintained.
Generally, a zoom lens needs to enable fast focusing and minimize an aberration variation during focusing. It is important to appropriately configure the zoom type, the focus lens unit, and lens units preceding and subsequent to the same to ensure high optical performance over an entire object distance.